1. Field of the Invention
The present invention relates to output waveguides for transmitting electromagnetic energy from a microwave amplification device, and more particularly, to a novel leaky wall filter for suppressing the 2.pi. mode oscillation within an output waveguide of an extended interaction klystron.
2. Description of Related Art
Linear beam tubes are used in sophisticated communication and radar systems which require amplification of an RF or microwave electromagnetic signal. A conventional klystron is an example of a linear beam microwave amplifier. A klystron comprises a number of cavities divided into essentially three sections: an input section, a buncher section, and an output section. An electron beam is sent through the klystron, and is velocity modulated by an RF electromagnetic input signal that is provided to the input section. In the buncher section, those electrons that have had their velocity increased gradually overtake the slower electrons, resulting in electron bunching. The traveling electron bunches represent an RF current in the electron beam. The RF current induces electromagnetic energy into the output section of the klystron as the bunched beam passes through the output cavity, and the electromagnetic energy is extracted from the klystron at the output section. An output waveguide channels the electromagnetic energy to an output device, such as an antenna.
The development of high powered klystron amplifiers which operate at a peak power level higher in relation to pulse length and frequency than that of conventional klystrons has resulted in beam voltage levels generally higher than that previously achieved. To avoid RF breakdown in the output section due to the high beam voltage, multi-cavity output circuits were developed. The multi-cavity output circuits, known as extended interaction output circuits (EIOC), have the advantage that a higher level of impedance across a greater bandwidth can be achieved. The higher impedance enables better matching with the electron beam, leading to greater efficiency of operation. An EIOC used to produce high power microwave energy with large instantaneous bandwidth is referred to as an extended interaction klystron (EIK), and can be used to produce power over bandwidths in excess of ten percent. An example of a high performance EIOC is disclosed in U.S. Pat. No. 4,931,695, to Symons.
A significant drawback of the EIK results from the multi-cavity configuration of the EIOC. If the EIK is utilized in a system having a poor impedance match with an output device at a frequency directly above the operating band of the EIK, instability in the form of unwanted oscillations can occur. For example, the use of a rotary joint coupled to the output waveguide can present a poor impedance match above the operating band. A rotary joint enables the rotation of an antenna so that RF energy can be emitted throughout a circular range of motion. Rotary joints are optimally designed for a good impedance match at the operating band of frequencies of the EIK. It is generally difficult, however, to maintain the quality of the impedance match over a broad band of frequencies outside the operating band.
The cause of the unwanted oscillations is traceable to the 2.pi. mode of the EIOC. The 2.pi. mode is a resonant condition occurring when the gap voltages in each of the output cavities of the EIOC are exactly in phase. Under poor impedance match conditions, power can "feedback" from the second or downstream output cavity back to the first cavity through inductive coupling slots that couple the cavities. The feedback power can modulate the electron beam which, in turn, delivers the energy back to the subsequent gap or gaps in phase. Unless the 2.pi. mode frequencies encounter a good impedance match, the cycle would repeat and build up to the point of oscillation. In some extreme cases, 2.pi. mode oscillation can result in catastrophic damage to the EIK, the rotary joint or other output devices coupled to the EIK. Since the 2.pi. mode frequencies may occur beyond the operating band of the EIK, even EIKs having a good impedance match throughout the operating band may not be immune to the undesirable 2.pi. mode oscillation.
In other types of microwave devices, such as coupled cavity traveling wave tubes (TWTs), this problem has been successfully dealt with through the use of lossy dielectric resonators coupled to the sides of the cavity walls. Because a device comprised of many cavities will have a well defined 2.pi. mode frequency, the dielectric resonators may have a high value of quality factor (Q) in order to achieve an adequate degree of attenuation of the 2.pi. mode oscillation over a very narrow range of frequencies and must be trimmed to the precise frequency of the 2.pi. mode.
In a typical EIK, however, the range of frequencies of the 2.pi. mode is too broad to be successfully damped by the relatively narrow band dielectric resonators. The output circuit of the EIK is equipped with tuning adjustments in order to optimize power and bandwidth. In the course of this tuning, the various resonances within the EIOC, including the 2.pi. mode, will change frequency. This renders the use of internal, high-Q, fixed frequency dielectric resonators impractical for most EIK applications.
Accordingly, it would be desirable to provide an apparatus for use with an EIK that suppresses the 2.pi. mode oscillation over a relatively broad frequency range. It would be further desirable to provide an apparatus having the above characteristics, while being relatively simple to design and cost effective to fabricate.